With the popularity of the Internet, collaborative (a.k.a., groupware) applications are being used by millions of people every day to meet and interact in cyberspace. Such interactions not only complement traditional means of collaboration, but are also often used as a preferred means of interaction. For example, e-mail is accepted as a preferred mode of communication over traditional telephone calling for many situations.
Two categories of collaborations exist, asynchronous and synchronous. Asynchronous collaborative applications, which do not require collaborators to be available simultaneously, include electronic mail and web browsing. Simple synchronous collaborative applications such as chat rooms and whiteboards, have been available for a long time and are widely used by an increasing number of users over the Internet. Since these simple applications do not require support for audio or video, they can be easily developed using the state sharing and event sharing abstractions provided by most infrastructures for collaborative applications.
Prior art works have addressed the issue of stream synchronization during playback or presentation time. Nelson Manohar, et al, in an article entitled "The Session Capture and Replay Paradigm for Asynchronous Collaboration" from "The Proceedings of the European Conference on Computer-Supported Cooperative Work", pp 149-164 (September 1995), describes a system for recording a multimedia session of one collaborator to later be viewed and modified by another collaborator. A session consists of continuous media streams and discrete media streams all originating from a single user. A recorded session can be played back in the workspace of another user at a later point in time, with the system providing synchronization of streams during playback. However, existing solutions do not address those situations in which the streams are generated and consumed in real time in a multi-user collaborative setting. In such situations, there is an additional problem of maintaining consistency of replicas of shared streams given the existence of simultaneous multiple sources to an event stream. Traditional replica consistency schemes impose consistent order on the updates made to a replicated object. What is needed is a novel integration of replica consistency and stream synchronization. Another challenge in such systems is the need to provide a seek capability to "seek to" a point or transaction on a previously-provided stream. The aforementioned Manohar, et al system implements its seek capability on discrete media streams using a fast re-execution of data in the stream from the beginning to the seek point. It is desirable to provide an improved and efficient seek capability that requires re-execution of only a portion of the data.
Further, there are many interesting and useful collaborative applications such as ones for foil presentation, call center, and expert consultation, that require support for different media components (e.g., data, audio and video). These applications need middleware support for synchronizing media streams, as the latency of various streams can vary significantly. For example, in a typical foil presentation application, where a presenter shows a sequence of annotated foils accompanied with audio and/or video to a distributed group of attendees connected via the Internet, it is important that a specific foil and the corresponding annotation be displayed on a participant's screen at the same point in time as when the participant receives the audio and video describing the displayed foil. It is also desirable, therefore, to address issues related to multimedia collaboration.
The most common approach to implementing synchronization of media streams involves multiplexing the different streams into a single transport stream. A receiver demultiplexes the single transport stream, into different media streams, then feeds the streams to the corresponding players. There are several drawbacks to this approach. First, the prior art approach requires that the system use its own custom streaming protocol. As a result, the available and popular streaming technologies cannot be utilized. Second, the prior art approach makes the less communication less efficient. It is desirable to use multiple transport protocols so that noncontinuous data that is critical for the integrity of the applications can be sent using a reliable transport protocol; whereas continuous media streams can be sent using an unreliable transport protocol. For example, in a multimedia foil (a.k.a., transparency) presentation application, it is usually acceptable to drop a few packets in the audio stream. However, dropping data which encodes information synchronized to a foil can result in a mismatch between the foil displayed and the audio played at the remote client location. Thus, multiplexing the streams into one stream forces the use of a protocol that is less than ideally suited for its transport. Third, the sources of the streams for multimedia presentations are generally physically different machines. For example, in a foil presentation, a presenter can use a laptop for foils and the whiteboard and a different machine for capturing and encoding audio and video. Multiplexing streams generated by different machines can be difficult if not impossible. Moreover, the existing approaches require a globally synchronized clock which is difficult to maintain.
In addition to media synchronization, an infrastructure for multimedia collaboration must provide a powerful programming framework and a set of application programming interfaces (APIs) which an application developer can easily use to develop complex applications that include multimedia components. The infrastructure should ideally provide support for (1) archiving and replaying live collaborations, and (2) VCR-like controls (play, pause, seek, rewind etc.) in a live or archived collaboration.
Finally, it is desirable to enable existing data sharing components to work synchronously with other media streams in a complex application. For example, it would be desirable to extend an existing stand-alone foil viewer application for use in a multimedia foil presentation application here foil displays are synchronized with an audio presentation. Also, with an improved streaming technology in use, the foil viewer component should not need changes.
It is therefore an objective of the present invention to provide a novel integration of replica consistency and stream synchronization in collaborative environments.
It is another objective of the invention to imbue a collaborative system with an efficient seek capability that requires re-execution of only a portion of the data.
Yet another objective of the invention is to provide for synchronization of multimedia presentations wherein the most efficient transport protocol can be employed for each component stream.
Still another objective of the invention is to provide multimedia synchronization without requiring a globally synchronized clock.
A further objective of the invention is to provide an infrastructure for multimedia collaboration which includes a programming framework and APIs including support for VCR-like controls.
Another objective of the present invention is to implement the foregoing wherein existing material can be incorporated into a synchronized multimedia presentation.